There is much debate in the Naruto fandom over who is the true best friend of Naruto Uzumaki. The two main contenders for the spot tend to be Sasuke Uchiha or Shikamaru Nara. Sasuke was one of Naruto's first friends and his rival throughout the story, while Shikamaru helped Naruto after the death of Jiraiya and is his closest adviser.
Sasuke and Shikamaru are both trusted and close friends, but the reality is Sakura Haruno has always been Naruto's best friend.
Sakura is Naruto's Biggest Supporter
Ever since the Land of Waves Mission, Sakura has been an avid supporter of Naruto and his goals. She gives Naruto advice on chakra control, which allows him to quickly advance in his training. When Orochimaru attacks Team 7 in the Forest of Death, Sakura inspires Sasuke to fight by praising Naruto's willingness to preserve through impossible odds. During the final two parts of the Chunin Exams, Sakura is one of the only people to openly cheer for Naruto during his fight with Kiba and Neji.
Even when she is demoralized and depressed, Sakura makes sure to comfort Naruto first. After Sasuke defeats Team 7 and escapes during the second retrieval mission, Naruto is in despair over his inability to save a friend. Sakura reassures Naruto that they will get stronger and bring Sasuke back together. No matter what, Naruto can always count on Sakura to encourage him during his lowest moments.
Sakura Defends Naruto's Dreams
Sakura is one of the first characters in the series to truly believe in Naruto's dream of becoming Hokage. She has complete faith in Naruto and wants to see him accomplish his dreams almost as much as he does. During the first part of the Chunin Exams, Sakura almost forfeits so that Naruto won't be banned from taking the exams ever again. Part of the reason she decides to train under Tsunade is so she can better protect Naruto and his dream.
When Sakura is convinced that Naruto's promise to bring back Sasuke has become a burden, she attempts to give up on Sasuke. Sakura gives her false confession in the hopes that she can spare Naruto the pain of worrying about Sasuke and help him focus on his original dream. Sakura cares about Naruto so much that she's willing to let go of her own goals and wants so that he can be happy.
Sakura is Naruto's Wingman
Sakura recognizes Hinata's feelings for Naruto and actively tries to set the two of them up. Throughout The Last: Naruto the Movie, Sakura encourages Hinata to give Naruto the scarf she made him and to confess her feelings. She repeatedly creates opportunities for the two of them to be alone. When Naruto is concerned that Hinata might be in love with another person, Sakura reassures him that Hinata only has feelings for him. Sakura understands Naruto's feelings better than anyone else and wants him to live a happy life after all the pain and tragedy he has experienced. She is a key reason why Naruto and Hinata start their relationship in the first place.
Sakura Has Saved Naruto's Life More Than Anyone
Sakura is not only a defender of Naruto's dream but his life as well. During the second phase of the Chunin Exams, Sasuke and Naruto are badly injured and unconscious. When three Sound ninjas come to kill them, Sakura puts her life on the line to protect them. With the medical ninjutsu skills she learned under Tsunade, Sakura is even more effective at keeping Naruto alive. She physically keeps Naruto's heart beating when the Nine-Tailed Fox is ripped from him during the Fourth Shinobi War. Once again, when Naruto is critically injured by Toneri during The Last: Naruto the Movie, Sakura spends three entire days healing him. Without Sakura's impressive medical skills, Naruto wouldn't be around to save the world or achieve his dreams.
Sakura has shown fierce loyalty and dedication to Naruto throughout the series. She is his rock when he needs encouragement or support. She believes in his dream and will do anything to help him achieve it. She is defensive when others insult Naruto or his ambitions. Sakura has routinely put Naruto's feelings before her own feelings and wants. She always attempts to make decisions that she thinks are in Naruto's best interest, such as setting him and Hinata up. None of Naruto's other friends have done as much for him or protected him as much as Sakura. Clearly, Naruto couldn't ask for a better friend.